Merge pull request #30 from sorentwo/fault-tolerance

Introduce redis connection fault tolerance

CHANGELOG.md

@@ -1,3 +1,12 @@
+## Unreleased
+
+- Added: Global connection fault tolerance. Any Redis connection error will
+  raise be caught and a `nil` value will be returned instead. For `fetch`
+  operations that means the block will yielded, if provided.
+  [readthis#26][issue-26]
+
+[issue-26]: https://github.com/sorentwo/readthis/issues/26
+
 ## v1.1.0 2015-12-07
 
 - Fixed: Stop overwriting specific options with the default options. [issue-28].

README.md

@@ -137,6 +137,19 @@ sticky and a flag is stored with each cached value. If you subsequently go to
 deserialize values and haven't configured the same serializers in the same order
 your application will raise errors.
 
+## Fault Tolerance
+
+In some situations it is desirable to keep serving requests from disk or the
+database if Redis crashes. This can be achieved wiht connection fault tolerance
+by enabling it at the top level:
+
+```ruby
+Readthis.fault_tolerant = true
+```
+
+The default value is `false`, because while it may work for `fetch` operations,
+it isn't compatible with other state-based commands like `increment`.
+
 ## Differences From ActiveSupport::Cache
 
 Readthis supports all of standard cache methods except for the following:

lib/readthis.rb

@@ -16,4 +16,27 @@ module Readthis
   def serializers
     @serializers ||= Readthis::Serializers.new
   end
+
+  # Indicates whether connection error tolerance is enabled. With tolerance
+  # enabled every operation will return a `nil` value.
+  #
+  # @returns [Boolean] True for enabled, false for disabled
+  #
+  def fault_tolerant?
+    @fault_tolerant
+  end
+
+  # Toggle fault tolerance for connection errors.
+  #
+  # @param [Boolean] The new value for fault tolerance
+  #
+  def fault_tolerant=(value)
+    @fault_tolerant = value
+  end
+
+  # @private
+  def reset!
+    @fault_tolerant = nil
+    @serializers = nil
+  end
 end

lib/readthis/cache.rb

@@ -345,6 +345,8 @@ def invoke(operation, key, &block)
       instrument(operation, key) do
         pool.with(&block)
       end
+    rescue Redis::BaseError => error
+      raise error unless Readthis.fault_tolerant?
     end
 
     def extract_options!(array)

spec/readthis/cache_spec.rb

@@ -48,7 +48,7 @@
     end
 
     it 'uses a custom expiration' do
-      cache = Readthis::Cache.new(namespace: 'cache', expires_in: 86400)
+      cache = Readthis::Cache.new(namespace: 'cache', expires_in: 86_400)
 
       cache.write('some-key', 'some-value')
       cache.write('other-key', 'other-value', expires_in: 1)
@@ -166,6 +166,16 @@ def self.load(value)
       cache.write('great-key', 'great')
       expect(cache.fetch('great-key', nil)).to eq('great')
     end
+
+    it 'serves computed content when the cache is down and tolerance is enabled' do
+      Readthis.fault_tolerant = true
+
+      allow(cache.pool).to receive(:with).and_raise(Redis::CannotConnectError)
+
+      computed = cache.fetch('error-key') { 'computed' }
+
+      expect(computed).to eq('computed')
+    end
   end
 
   describe '#read_multi' do

spec/readthis_spec.rb

@@ -6,4 +6,16 @@
       expect(Readthis.serializers.marshals).to include(Marshal, JSON)
     end
   end
+
+  describe '#fault_tolerant?' do
+    it 'defaults to being false' do
+      expect(Readthis).not_to be_fault_tolerant
+    end
+
+    it 'can be enabled' do
+      Readthis.fault_tolerant = true
+
+      expect(Readthis).to be_fault_tolerant
+    end
+  end
 end

spec/spec_helper.rb

@@ -20,4 +20,8 @@
 
   config.order = :random
   Kernel.srand config.seed
+
+  config.before do
+    Readthis.reset!
+  end
 end